Undercoat layers are necessary in electrophotographic applications to smooth any surface imperfections of the substrate, to prevent charge injection disruption of the thin and delicate charge generating layer and the like. Failure to deposit this layer results in image defects, such as black or white spots, depending upon the development process. Depending upon the substrate, dark decay may also be a problem. To be effective, the solvent used to coat the subsequent layers must not attack the undercoat layers. The best way to achieve this is to cross-link the undercoat layer once it has been deposited on the substrate.
For dip coating processes, coating solution stability and uniformity is a must. The cross-linking chemistry cannot be allowed to occur in the coating tank. Urethane chemistry has been used extensively in the automotive industry in cross-linked systems. In some applications, the isocyanate groups and the polyol second components are kept isolated and mixed immediately before coating. The isocyanate groups are very active. Once the two components are mixed, the pot life is very short. Blocking technology was developed to overcome these limitations. Blocked isocyanates have been used to generate cross-linked urethanes upon deblocking of the isocyanate moiety by heat. Thus, at room temperature the mixture is extremely stable. The blocked isocyanates use a variety of blocking agents to achieve a range of activation temperatures and physical properties. Methyl ethyl ketoxime unblocks at a temperature of about 150° C. Caprolactam unblocks at 170° C. Alkylated phenol unblocks at temperatures above 170° C. and malonic acid esters unblock at 90° C.
The deblocking temperatures are minimum deblocking temperatures. In actual practice, the reaction temperature depends on the structure of the isocyanate and the presence of catalysts.
For electrophotographic applications, dip coating is preferred for the manufacture of high quality, photosensitive drums. Other coating techniques may be used such as hopper coating, spray coating, knife coating and the like. All such coating techniques require that the coating solution be stable and uniform. To optimize throughput and reduce cost, it is highly desirable to minimize processing time and energy. It is desirable to choose chemistries, including cross-linking chemistries, which are compatible with efficient processing. Desirably, the curing time is very rapid and lower temperatures for curing are preferred.
Further, electrophotographic systems are very sensitive to impurities. The wrong materials, even at parts per million (ppm) levels, can be very problematic. Therefore, it is essential to use the purest materials available as well as avoiding generating harmful byproducts during the electrophotographic element processing, especially during the curing step. This consideration must be taken into account in choosing the cross-linking chemistry to be used.
In U.S. Pat. No. 5,202,406, issued to Randall S. Dearth, et al. on Apr. 13, 1993, a ketoxime blocked poly isocyanate and cyclohexanedicarboxylic acid-based polyester polyol sprayable coating composition is disclosed. The disclosed composition is useful as an automotive chip resistant primer surface polyurethane coating. Oxime blocked poly isocyanates have been disclosed in U.S. Pat. Nos. 4,474,934, issued to Michio Tanaka, et al. on Oct. 2, 1984; 4,533,684, issued to Panagiotis I. Kordomenos on Aug. 6, 1985; 4,596,744, issued to Terry L. Anderson, et al. on Jun. 24, 1986; 4,785,068, issued to Akira Tominaga, et al. on Nov. 15, 1988; 4,806,585, issued to Takeo Nakayama, et al. on Feb. 21, 1989; and 4,824,925, issued to Peter Kamarchik, Jr., et al. on Apr. 25, 1989. The use of Oxime blocked isocyanates for electrophotographic applications is not preferred for two reasons. One the deblocking temperature is very high, 150° C. Two, the Oxime byproduct has the potential to interfere with electrophotographic performance. U.S. Publication No. 2004/0030086 A1, by Michael Schelhaas, et al. published on Feb. 12, 2004 also reported that certain Oxime blocking agents such as butanone Oxime are toxicologically objectionable.
Similarly caprolactam and alkylated phenol blocked isocyanates are not preferred for electrophotographic applications, because of their high deblocking temperatures.
U.S. Publication No. 2003/0236360 A1, by Gurtler et al., published Dec. 25, 2003, disclosed lower temperature blocked isocyanates incorporating secondary amines such as tert-butyl-benzylamine. These blocked isocyanates deblock at about 120° C. However for electrophotographic applications, it is expected that the secondary amine byproduct, even at ppm level can act as a trap.
U.S. Pat. No. 5,204,203, issued to Yoshimasa Ito, et al. issued Apr. 20, 1993, describes the use of Oxime or caprolactam based blocked isocyanates in a single-layer photoconductor. Curing was carried out for four hours at 150 to 160° C.
Baxenden Chemicals, Limited, Baxenden, England, developed the use of 3,5-Dimethylpyrazole (DMP). Several advantages of DMP blocked isocyanates are cited, including lower unblock temperature (110-120° C.) and increased cure response. These attributes would make DMP blocked isocyanates ideal material for electrophotographic applications. However another cited advantage of DMP agent is its low volatility, which results in reduced volatile organic compounds (VOC) emissions. The DMP tends to stay in the coating. The chemical structure of DMP is such that interference with electrophotographic performance is most probable
The foregoing patents and publication are hereby incorporated by reference in their entirety.
While it is clear that a variety of blocking materials have been used with isocyanates, it is also clear that the isocyanate-blocked materials present certain problems with respect to production of electrophotographic elements. These problems relate to high unblocking temperature and the generation of undesirable materials that may remain in the polymeric layer. Accordingly, a continued effort has been directed to the development of methods for producing smooth layers of polyurethane by use of low unblocking temperatures and which contain substantially no residual materials as a result of the unblocking process.